Initially, offshore power plants such as offshore wind parks for generating multi-phase AC power were connected to an on-shore grid over a high-voltage alternating current (HVAC) transmission link. However, developments in the field of high-voltage direct-current (HVDC) transmission are making it more likely that, in future, more offshore wind power plants will be connected to a main grid over a HVDC transmission connection.
To feed into a collector network, the AC power (usually three-phase) of a wind turbine or other generator generally undergoes an AC-DC conversion and then a DC-AC conversion before being fed into the collector network via a transformer. These steps can be performed by a converter or voltage source converter (VSC), generally comprising semiconductor power switches such as isolated gate bi-polar transistors (IGBTs) and gate control circuits. At a point of common connection (PCC) in the collector network, the AC power is fed into a VSC for AC-DC conversion prior to transmission over the HVDC link. At the other end, another VSC converts the DC power into multi-phase (usually three-phase) AC power for feeding into the grid.
A fault occurring at the grid end can either be symmetrical (i.e. the voltage in all phases collapses evenly) or asymmetrical (only one or two voltage phases collapse). A symmetrical fault is usually referred to as a “low voltage fault”, while an asymmetrical fault is generally referred to as a “single line-to-ground fault”. Asymmetrical faults are the most common types of fault that occur in multi-phase (usually three-phase) power transmission systems. The ability to quickly recover from an asymmetrical fault and to resume normal operation is usually a grid requirement. Also, most grid requirements specify that a power plant should be able to stay connected to the grid during a short-term fault and to continue supplying reactive power for the duration of the fault, and for this reason, a power plant is usually designed with a “fault-ride-through” mechanism.
When an asymmetrical fault occurs at the grid side of a HVDC system, the active power consumed by the grid will decrease, while the power plant continues to generate active power. In order to stay connected to the grid during the fault, the excess active power—manifesting as an excess DC voltage—must be dealt with in some way at the power plant side. The most common way of handling the problem of excess DC voltage is by dissipating the excess active power using a DC chopper. For example, the increase in DC voltage can be controlled by loading a power resistor while the power plant continues to produce active power as it did before the asymmetrical fault occurred, so that the active power transferred to the grid is reduced. This means that the power plant can continue to operate regardless of the disturbance at the other end of the HVDC transmission line. However, including a DC chopper adds to the cost of a wind park, particularly in the case of a large wind park comprising many wind turbines, and is inefficient from an energy conservation point of view, since great effort and cost must be invested in efficient heat sinks for cooling the DC chopper.
In a HVDC transmission arrangement, a power plant is effectively de-coupled from the main grid. Therefore, an AC disturbance such as an asymmetrical fault occurring at the main grid will not be directly “seen” by the wind power plant. In an alternative approach, the decrease in active power transfer to the grid is measured and passed on to the power plant using communication signals. For example, in the case of a wind power plant with a plurality of wind turbines, the individual wind turbines will receive new power setpoints relating to a lower production, in order to obtain a new power balance until the system has recovered from the fault. However, the disadvantage of this approach is that it takes relatively long time for the power plant to react to the asymmetrical fault, since the asymmetrical fault must first be communicated in some way to the power plant over a communications interface, and it takes a correspondingly longer time for the situation to stabilize.